This invention concerns a fifth wheel coupling for mounting on a towing vehicle for co-operation with a king-pin mounted on a trailer.
One known form of fifth wheel coupling is disclosed in GB2236729. Another known form of fifth wheel coupling is disclosed in the applicants"" own earlier GB2 342 334. In both of these cases the coupling comprises a plate having a slot for reception of the king-pin, a pivotally mounted jaw arranged adjacent the slot and movable between an open position and a closed position in which it can co-operate with the king-pin to retain it in the slot, and a locking member displaceable between a locking position, in which it serves to maintain the jaw in its closed position, and a release position in which it allows the jaw to open. The locking member is itself connected to a pivotally mounted linkage mechanism which includes a release arm. The release arm is engageable with the plate in a first position in which the linkage mechanism holds the locking member in its release position and in a second position in which the linkage mechanism holds the locking member in its locking position.
As the king-pin enters the slot it causes the jaw to pivot into its closed position. This releases the locking member so that it is urged into its locking position by spring means acting on the linkage mechanism. At the same time this brings the release arm into engagement with the plate in its second position.
In the coupling shown in GB2236729 an additional locking device is provided in the form of a spring snap hook which is manually engaged through aligned apertures in the release arm and the plate. This is to ensure that the release arm remains in engagement with the plate in its second (locking) position and is not inadvertently dislodged while the coupling is in use.
It is desirable that movement of the release arm to its first position and the associated linkage mechanism, including the locking member, to its release position should be easy to accomplish, when required, with minimum manual effort.
An object of the invention is to provide an improved fifth wheel coupling which incorporates this feature.
With this object in view, the invention provides a fifth wheel coupling for mounting on a towing vehicle for co-operation with a king-pin mounted on a trailer, the coupling comprising a plate having a slot for reception of the king-pin, a pivotally mounted locking mechanism whereby the king-pin can be retained in the slot, said mechanism including a release arm which is engageable with the plate in a first position, wherein the mechanism is held in its release position, and in a second position, wherein the mechanism is held in its locking position, and a fluid actuatable piston and cylinder arrangement, the piston of which is connected to the release arm, in which respect the piston is actuatable to move the release arm, against spring bias, into its first position, whereby the locking mechanism is capable of releasing the king-pin.
Switch means for actuating the piston and cylinder arrangement may conveniently be mounted on the release arm.
In practical embodiments a plunger may be slidably mounted on the release arm to serve as an additional locking device maintaining the release arm in engagement with the plate in its second position, and this plunger may advantageously serve the further purpose, as it slides relative to the release arm, of actuating the piston. In this respect, a suitable practical arrangement may be for the plunger to be spring biased into a first position in which it acts as the additional locking device and to be withdrawn against spring bias until it actuates the switch means for power assisted release of the coupling.
Engagement of the release arm with the plate in the first and second positions referred to may conveniently be by respective recesses or detents in the release arm engaging the edge of a slot in the plate. The plunger may then serve as an additional locking device by being arranged for projection through the slot in addition to the release arm. Preferably it is projectable through the slot only once engagement of the respective release arm recess holding it in its second position has been accomplished.
In this respect, a stop block may be mounted adjacent the slot so as to block entry of the plunger into the slot until the said recess has engaged there with. Such entry or non-entry of the plunger into the slot is preferably visible to a user of the coupling so that it is apparent whether or not reliable locking of the coupling has been achieved.